1. Field of the Invention
The invention is in the field of photolithographic processing of solid state devices.
2. Brief Description of the Prior Art
As the demand for higher package densities in solid state microcircuits has increased, the use of X-rays has been investigated as a means of reducing the diffraction limitation inherent in photolithographic processing in the visible and near visible spectral region (D. L. Spears et al, Solid State Technology, 5 (1972) 21-26). The exposure of photolithographic resist materials at the necessarily small scale has been accomplished by electron beam scanning. This method is satisfactory for the production of shadow masks, however, it is generally too slow for most production uses. X-rays in the 1 A to 100 A spectral region have sufficiently short wavelengths to overcome diffraction limitations in the production of microcircuits requiring element sizes in the micrometer to submicrometer range. The investigation of X-ray lithographic processes has included consideration of X-ray sources possessing sufficient intensity for the exposure of the photolithographic resist materials within reasonable production times. Another consideration is the provision of shadow mask materials and absorber layer thicknesses producing sufficient contrast in the exposed resist to permit further device processing. The collimation of the X-ray beam must also be considered since it affects the accurate reproduction of the pattern contained in the shadow mask.
The most common sources of X-ray radiation are vacuum tubes within which a beam of electrons, accelerated to the order of 20,000 volts is directed at a metal target. The emitted X-ray radiation contains one or more intense narrow lines, known as the characteristic radiation of the target, and a broad continuous background. The most thoroughly investigated X-ray lithographic process utilize the characteristic radiation. The selection of photomask absorber material and X-ray target material is governed by the requirement that the photomask material be highly absorptive at the wavelength of characteristic radiation of the X-ray target material. It has been found, however, that although the background radiation is much lower in intensity per unit bandwidth than the characteristic radiation, it extends over regions of the spectrum in which the shadow mask absorber layers are not as highly absorptive. Thus exposure of the resist material by background radiation tends to reduce the contrast of the exposed pattern. This requires, for example, the use of thicker layers of absorptive material than would be required if the characteristic radiation alone were present. The use of the thinnest possible absorber layers is desirable for producing patterns for higher and higher resolution.
Standard X-ray sources emit their radiation over a large solid angle. In using this radiation a balance must be reached between the desire to use as much of the emitted radiation as possible, in order to minimize exposure times, and the desire to limit the variation of incidence angle across the exposed sample. This angular limitation is accomplished by collimation of the beam and placement of the device to be exposed further from the X-ray source. This however is at the expense of reduced intensity and longer exposure time.
Synchrotron radiation from a particle accelerator is a known intense source of X-ray radiation. However the emission is over a broad continuous spectrum, which raises the questions of the reduced contrast treated above. Various filtering schemes utilizing selective absorption of unwanted portions of the X-ray spectrum have been considered. However such schemes are limited in their ability to provide high selectivity together with low absorption of the desired radiation.